Critical Mass

Critical Mass (aka Critter) is a shoot'em up (smup) similar to
Galaxian.

The basic idea is simple: you move your ship back and forth at
the bottom of the screen, shooting up, while bad guys fly
around and shoot at you.

Your world has been infested by an aggressive army of space
critters. Overrun and
unprepared, your government was unable to defend
its precious resources. As a last effort to
recapture some of the 'goodies', you have been
placed into a tiny spacecraft and sent after them.